1. Field of the Invention
This invention relates to a process for chromating zinc surfaced steel objects to improve the resistance of the chromate coating formed to leaching by conventional aqueous based degreasing compositions, and to chromating solutions useful for such a process. The invention is applicable, for example, to all varieties of electroplated and/or dip coated forms of galvanized steel or zinc alloy coated steel, when the surface coating layer is metallic and is at least half zinc by weight. The film formed over the zinc surface has chromic acid or chromate as its principal component. This invention is particularly applicable to sheets and other flat zinc surfaced objects intended for later shaping into articles for ultimate use.
2. Statement of Related Art
It is known that the pre-painting and post-painting corrosion resistance of zinc surfaced steel objects may be improved by the formation of a chromate film on the objects, resulting from application to and drying on the sur faces of the objects of an acidic aqueous solution having chromic acid or chromate as its principal component. The chromium add-on in the chromate film formed on such a surface is generally from 5 to 200 milligrams per square meter ("mg/m.sup.2 "), and the object is normally dried at temperatures of 60 to 150 degrees Centigrade. Steel sheet carrying the chromate film generated by such a treatment is then normally subjected to cutting and/or forming operations and subsequently painted after such steps as degreasing, rinsing, and the like.
With chromate films obtained by the methods now conventional in the art, part of the chromate film elutes into the degreasing solution during a conventional degreasing step, and this compromises the performance quality of the film. In addition, this eluting portion of the prior art chromate films is predominantly hexavalent chromium, and its contamination of the degreasing solution is disadvantageous because of the risk of environmental pollution.
One means for inhibiting this chromium elution is to increase the trivalent chromium content in the chromate coating solution. However, because a chromate coating solution can easily gel as the chromate coating process progresses, as a result of build up in the coating solution of eluted zinc and trivalent chromium produced by reduction, there are practical limitations on the trivalent chromium content that can be produced in the films, without causing instability of the chromate coating solutions.
Accordingly, the prior chromate coating solutions suffer from problems in terms of avoiding environmental pollution and/or coating solution stability.